(1) Field of the Invention
The present invention relates to a water-in-oil (W/O) emulsion ink which is suitable for use in inkjet printing and is low in viscosity and superior in storage stability.
(2) Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
Inkjet printing is characterized by ejecting ink droplets from fine nozzles so as to perform non-contact printing. Companies have carried out R & D for inks for inkjet printing, aiming at making ink droplets finer and making printing speed faster and printing area larger.
As an ink used for inkjet printing (referred to as “ink for inkjet” in the present application), an aqueous pigment or dye ink is generally used for personal and office uses. Because the aqueous pigment or dye ink is easy to be absorbed into paper fibers, it is high in printing density and low in strike through which is a printing density looked from the backside of prints. On the other hand, since paper is dried under pressure during production, hydrogen bonds between the paper fibers are cut and cause curl when water contacts the paper. This curl is so remarkable as to influence accuracy of positioning of ink droplets and conveyance of paper when cut sheets of paper are used. It is prerequisite to eliminate the curl in order to achieve high speed printing.
As a technique for eliminating the curl, reducing the water content of the ink or removing water from the ink may be employed. In other words, oil based inks can be used to eliminate the curl, which are also suited for high speed printing.
Business printers which adopt a line head type inkjet system are noticed since they have the head fixed so as to produce a large amount of prints at high speed and are available at low price. An oil based ink is usually used for this high speed inkjet printer. However, the ink permeates into the backside of paper after printing, and thus the resulting prints are low in printing density with lots of strike through, compared to printers adopting other systems.
Emulsification of ink is one of the methods for solving this problem (refer to patent document 1). Although emulsification of ink for inkjet can increase printing density and decrease strike through, ink becomes too thick to be ejected from nozzles as droplets when a percentage of a water phase is increased. Moreover, when the ink is inferior in storage stability, it affects ejection of ink so that printing cannot be performed even though the ink viscosity is controlled to be low.
Patent Document 1: JP-A-2006-56931